Should I reserve a seat during busy season

There are 5 of us travelling from Luzern to Interlaken at the end of July. I believe we do not need to reserve seats on the Golden Pass train but as there will be so many tourists at the time, would it be better for me to reserve a seat or will it not be a problem to just show up?

Another train question - we are planning to take the cable car to Mannlichen, walk to Kleine Scheidegg and then catch the train to Jungfrau. I hear the Jungfrau train gets pretty crowded as the day progresses. Any idea when the busiest time would be? I'm debating if we should scrap this plan and just do Jungfrau first or if we will be ok if we get to KS by noon at the latest.

Although many consider the walk in the reverse direction to be OK ( KS to Männlichen) which could be done after Jungfraujoch, I personally feel very strongly that it should be done in the other direction (M to KS)..

M to KS is slightly down hill ...not a big issue.

However, one of the special parts of the trip from M to KS is the way that the Eiger rises above you as you walk toward it. If you go the other way, you have to keep turning your head around, and the effects is not nearly as compelling.

Old picture is attached, from 2004. A crop from it shows the hikers at full image size...gives you some idea of how insignificant people are in that landscape.

Thank you, Arno & Slowpoke. I definitely want to do the walk from M to KS and not the other way around. I am just concerned about having to wait too long for a train to Jungfrau - we are in the region for such a short time, I don't want to waste valuable time that could better be spent enjoying the area. There is so much I want to do and so little time :-'(